The earliest legend about the Loch Ness monster
recorded is the story of St. Columba's encounter with the beast.

Guilty for being partly responsible for the death of many men
in the Battle of Cul-drebene, St Columba set out to mainland Scotland
on a pilgrimage to spread Christianity across the land.

During this time, on his way to visit with the Pictish king in
Inverness, he encountered some Picts burying what remained of
one of their own people - badly savaged by a creature in the Loch.

The dead mans boat lay on the other side of the water, so Columba
ordered one of his followers to swim over and retrieve the boat.

During this the servant was attacked
by a creature that reared out of the Loch to attack the swimmer.

Columba (invoking the name of God) commanded the beast to return
to whence it came and it vanished beneath the waters of the Loch
leaving the swimming man unharmed.

Of course, this "true account" was written almost a
century after the events took place and like most Christian legends
is open to mis-interpretation and deliberate exaggeration (and
is only one of many outlandish "miracles" attributed
to the man).